Powder coating, process for the exterior coating of metal pipes, and use of the powder coating for the one-layer exterior coating of metal pipes

ABSTRACT

Powder coating, process for the exterior coating of metal pipes, and use of the powder coating for the single-layer exterior coating of metal pipes. The powder coating comprises epoxy resins, phenolic crosslinking agents, catalysts, fillers and additives. The fillers used are crystalline silicic acids modified with glycidyl groups. The invention also relates to a process for the exterior coating of metal pipes and to use of the coatings for a single layer exterior coating of metal pipes.

The present invention relates to powder coatings containing epoxyresins, phenolic crosslinking agents, catalysts, fillers and, ifdesired, auxiliaries and additives. The invention further relates toprocesses for the exterior coating of metal pipes using these powdercoatings, and to the use of the powder coatings for the one-layerexterior coating of metal pipes.

Nowadays, metal pipes are usually protected against corrosion by beingcoated with reactive powder coatings. It is known to use for thispurpose powder coatings based on epoxy resins and suitable crosslinkingagents. The powder coatings which are suitable for this one-layercoating process for metal pipes must meet stringent requirements withregard to protection against corrosion, hot water exposure and cathodicdelamination. Powder coatings suitable for exterior coating of pipes areknown, for example, from EP-B-104 719 and U.S. Pat. No. 4,122,060. Thepowder coatings known from EP-B-104 719 are epoxy resins which arecrosslinked using, for example, phenolic curing agents. The powdercoatings described also contain catalysts, and calcium oxide as filler.

U.S. Pat. No. 4,122,060 describes powder coatings based on epoxy resins,curing agents, fillers and catalysts, the filler employed preferablybeing amorphous silicic acids. Furthermore, it is known to employfelspars and barytes or alternatively precipitated barium sulfate asfillers in epoxy powder coatings for pipe coating.

The previously known powder coatings based on epoxy resins, suitablecrosslinking agents, catalysts and fillers such as, for example,amorphous silicic acids, felspars, barytes and precipitated bariumsulfates, have the disadvantage that they exhibit poor properties withrespect to resistance to hot water and to cathodic delamination (DIN30671).

The object of the present invention was therefore to eliminate thedisadvantages of the prior art by providing powder coatings which arebased on epoxy resins, on suitable crosslinking agents such as, forexample, phenolic curing agents, on fillers and on catalysts and whichare suitable for the exterior coating of metal pipes. The pipes coatedwith these powder coatings should have good protection againstcorrosion, improved resistance to hot water and improved resistance withregard to cathodic delamination. Furthermore, the coatings obtainedshould be of high flexibility.

The object of the invention is achieved by powder coatings comprisingepoxy resins, phenolic crosslinking agents, catalysts, fillers and, ifdesired, auxiliaries and additives. The powder coatings arecharacterized in that the fillers used are crystalline silicic acidmodifications which are functionalized with glycidyl groups.

Suitable epoxy resins are all solid epoxy resins having epoxideequivalent weights of between about 400 and 3000. These arepredominantly epoxy resins based on bisphenol A and bisphenol F.Particular preference is given to employing epoxidized novolak resins.Also suitable are mixtures of bisphenol A or bisphenol F resins andnovolak resins. The epoxy resins based on bisphenol A and bisphenol Fpossess, in general, a functionality<2, the epoxidized novolak resins afunctionality>2. The resins used with particular preference in thepowder coatings according to the invention are epoxidized novolakshaving an average functionality in the range from 2.4 to 2.8 and with anepoxide equivalent weight in the range from 600 to 850. In theepoxidized novolak resins the phenolic hydroxyl groups are etherifiedwith alkyl, aryl or similar groups. By reacting the phenolic hydroxylgroups with epichlorohydrin, epoxide groups are incorporated into themolecule. Starting from novolaks, the so-called epoxy novolak is formedin this process. The epoxidized novolaks are structurally related tobisphenol A resins. Epoxidized novolak resins can be prepared byepoxidation of novolaks consisting, for example, of from 3 to 4 phenolrings which are joined to one another via methylene bridges. The novolakresins used can also be alkyl-substituted phenols which are reacted withformaldehyde.

Examples of suitable epoxy resins are the products which arecommercially available under the following names:

Epikote 154, 1001, 1002, 1055, 1004, 1007, 1009, 2014, 3003-4F-10 fromShell-Chemie, XZ 86795 and DER 664, 667, 669, 662, 642U and 672U fromDow and Araldit XB 4393, XB 4412, GT 7072, GT 7203, GT 7004, GT 7304, GT7097, GT 7220 and GT 7255 from Ciba Geigy.

For the curing of the epoxy resins the powder coating according to theinvention contains phenolic cross-linking agents. In this context, forexample, any desired phenolic resin can be used provided it has themethylol functionality necessary for reactivity. Preferred phenolicresins are reaction products, prepared under alkaline conditions, ofphenol, substituted phenols and bisphenol A with formaldehyde. Underthese conditions the methylol group is linked to the aromatic ringeither in the ortho or the para position. Particularly preferredphenolic crosslinking agents in accordance with the present inventionare hydroxyl group-containing bisphenol A or bisphenol F resins having ahydroxyl equivalent weight in the range from 180 to 600, particularlypreferably in the range from 180 to 300. Such phenolic crosslinkingagents are prepared by reacting bisphenol A or bisphenol F withcomponents containing glycidyl groups, for example the diglycidyl etherof bisphenol A. Such phenolic crosslinking agents are obtainable, forexample, under the trade name DEH 81, DEH 82 and DEH 87 from Dow, DX 171from Shell-Chemie and XB 3082 from Ciba Geigy.

In this context the epoxy resins and the phenolic crosslinking agentsare employed in a ratio such that the number of epoxide groups to thenumber of phenolic OH groups is approximately 1:1.

The powder coatings according to the invention contain one or moresuitable catalysts for curing the epoxy resin. Suitable catalysts arephosphonium salts of organic or inorganic acids, imidazole and imidazolederivatives, quaternary ammonium compounds and amines. The catalysts aregenerally employed in proportions of from 0.001% by weight to about 10%by weight, based on the total weight of the epoxy resin and of thephenolic crosslinking agent.

Examples of suitable phosphonium salt catalysts areethyltriphenylphosphonium iodide, ethyltriphenylphosphonium chloride,ethyltriphenylphosphonium thiocyanate, the complex ofethyltriphenylphosphonium acetate with acetic acid,tetrabutylphosphonium iodide, tetrabutylphosphonium bromide and thecomplex of tetrabutylphosphonium acetate with acetic acid. These andother suitable phosphonium catalysts are described in, for example, U.S.Pat. No. 3,477,990 and U.S. Pat. No. 3,341,580.

Examples of suitable imidazole catalysts are 2-styrylimidazole,1-benzyl-2-methylimidazole, 2-methylimidazole and 2-butylimidazole.These and other imidazole catalysts are described in, for example,Belgian Patent No. 756,693.

In some cases, commercially available phenolic crosslinking agentsalready contain catalysts for crosslinking the epoxy resin.

The powder coatings according to the invention are characterized in thatthey contain as filler crystalline silicic acid modificationsfunctionalized with glycidyl groups. These fillers are conventionallyemployed in a proportion of from 10 to 50% by weight, based on the totalweight of the powder coating. In some cases proportions of filler ofmore than 50% by weight are also possible.

The crystalline silicic acid modifications include quartz, cristobalite,tridymite, keatite, stishovite, melanophlogite, coesite and fibroussilicic acid. The crystalline silicic acid modifications arefunctionalized with glycidyl groups, this glycidyl-groupfunctionalization being achieved by a surface treatment. Examples ofthese silicic acid modifications are those based on quartz, cristobaliteand fused silica, which are prepared by treating the crystalline silicicacid modifications with epoxy silanes. The glycidyl-group functionalizedsilicic acid modifications are commercially available, for example,under the name Silbond® 600 EST and Silbond® 6000 EST (manufacturer:Quarzwerke GmbH).

The powder coatings according to the invention advantageously containfrom 10 to 40% by weight, based on the total weight of the powdercoating, of crystalline silicic acid modifications functionalized withglycidyl groups.

The powder coatings may also contain other inorganic fillers, forexample titanium dioxide, barium sulfate and silicate-based fillers suchas, for example, talc, kaolin, magnesium and aluminum silicates, micaand the like. In addition, the powder coatings may also, if desired,contain auxiliaries and additives. Examples of these are levelingagents, flow aids and deaerating agents, for example benzoin.

The powder coatings are prepared by known methods (cf. eg. the productinformation publication from BASF Lacke+Farben AG, "Pulverlacke" [Powdercoatings], 1990) by homogenizing and dispersing, for example using anextruder, screw kneading device and the like. After the powder coatingshave been prepared they are adjusted to the desired particle sizedistribution by grinding and, if appropriate, by classifying andseiving.

The powder coatings are applied electrostatically ortriboelectrostatically to the preheated metal pipe surface.

The invention therefore also relates to a process for the exteriorcoating of metal pipes using the above-described powder coatings. Inthis process the metal pipe surface is first usually cleaned to removerust, fat, oil, dust etc. If desired, a chemical pretreatment(chromatizing and/or phosphatizing) is carried out. Subsequently, thecleaned metal pipes are heated by inductive heating or in a gas oven toa coating temperature of from approximately 170 to 250° C. The powdercoatings according to the invention are applied to the hot surface ofthe metal pipe by electrostatic means or by triboelectrification. Theconventional thicknesses in which the powder coating is applied are inthe range from 100 to 1000 μm, preferably within the range from 300 to500 μm.

The powder coatings are cured within a few minutes.

The present invention furthermore relates to the use of theabove-described powder coatings for the exterior coating of metal pipes.

The coated metal pipes which can be obtained by the process according tothe invention exhibit outstanding properties. For instance, there is nodelamination of the powder coating from the substrate. The resistance tohot water of the powder coatings obtained is excellent, and the resultsof the CD test, which tests the resistance of the powder coating withrespect to cathodic delamination in accordance with DIN 30671, areoutstanding.

In the text below, the invention is illustrated in more detail usingexamples. In these examples parts are by weight unless otherwiseindicated.

    ______________________________________                                        Powder coatings are prepared having the following composition:                                       Com-     Com-                                                           Exam- parison  parison                                                        ple 1 Example 1                                                                              Example 2                                     ______________________________________                                        Pigment (parts by weight)                                                                        2.0     2.0      2.0                                       Blanc finxe-N [sic] (parts by weight)                                                            --      19.0     --                                        Minex 4 (parts by weight)                                                                        --      --       19.0                                      Silbond 6000 EST   19.0    --       --                                        (crystalline silicic acid functionalized                                      with glycidyl groups) (parts by weight)                                       Additives (parts by weight)                                                                      1.66    1.66     1.66                                      Catalyst, EPON P104 from Shell (parts                                                            0.5     0.5      0.5                                       by weight)                                                                    Phenolic crosslinking agent, XD 8062                                                             13.8    13.8     13.8                                      from Dow (parts by weight)                                                    Epoxy resin, trade designation DER                                                               31.0    31.0     31.0                                      664U from Dow                                                                 Novolak resin, trade designation DER                                                             32.0    32.0     32.0                                      672U from Dow                                                                 Aerosil R 972 from Degussa as flow aid                                                           0.04    0.04     0.04                                      ______________________________________                                    

The powder coatings of Example 1 and of Comparison Examples 1 and 2 areprocessed to give powder coatings of standard commercial particle-sizedistribution.

The powder coatings prepared are employed in the one-layer process forthe exterior coating of metal pipes. For this purpose, pipes of diameter300 mm with a wall thickness of 12 mm are blasted in a blasting unit todegree of cleanness SA 3. The peak-to-valley height should be about 50μm. The pipes are then heated to 230±5° C. using an induction coil. Thepowder coatings of Example 1 and of Comparison Examples 1 and 2 areapplied electrostatically at a coat thickness of approximately 100 μmand are cured.

The test results are compiled below.

    ______________________________________                                                            Comparison Comparison                                                 Example 1                                                                             Example 1  Example 2                                      ______________________________________                                        Gel time.sup.1)                                                                             59 sec    56 sec     58 sec                                     180° C.                                                                Indentation and.sup.2)                                                                      8.0 +     7.9 +      8.3 +                                      Flexure (table edge)                                                          on 0.5 mm steel                                                               panel, 10' 180° C.                                                     Coat thickness (μm)                                                                      60-70     80-90      80-90                                      Gloss 60°                                                                            87 E      90 E       75 E                                       Flow 10' 180° C.                                                                     moderate  moderate   moderate                                   Flexure.sup.3)                                                                RT            32 mm     12 mm      15 mm                                                    32 mm     10 mm      13 mm                                      approx. 0°                                                                           18 mm      6 mm      11 mm                                                    18 mm      7 mm      10 mm                                      ______________________________________                                         .sup.1) : determined using Coesfeld gel time instrument                       .sup.2) : 0 = cracks; * = hairline cracks; + = satisfactory                   .sup.3) : on 5 mm panel blasted with Wheelabrator GH 40, SA 2.5 Preheated     at 240° C., coated, afterbaked at 240° C. for 120 sec and       immediately cooled in water: coat thickness 400-500 μm. The higher the     flexure values, the higher the flexibility.                              

    ______________________________________                                        Impact on 10 mm panel blasted with Wheelabrator GH 40, SA 2.5,                preheated at 240° C., coated, after-baked at 240° C. for        120 sec.                                                                      and immediately cooled in water:                                                                 Com-           Com-                                                           parison        parison                                     Example 1          Example 1      Example 2                                   Layer              Layer          Layer                                       thickness  Imp..sup.1)                                                                           thickness                                                                              Imp..sup.1)                                                                         thickness                                                                            Imp..sup.1)                          lm         kg*cm   lm       kg*cm lm     kg*cm                                ______________________________________                                        Point 1                                                                             550      (60)    670    (50)  540    (50)                               Point 2                                                                             570      55      660    (45)  560    40                                 Point 3                                                                             570      (60)    660    (40)  560    45                                 Point 4                                                                             610      55      670    35    570    (50)                               Point 5                                                                             550      (55)    650    (40)  540    45                                 Point 6                                                                             600      55      640    35    610    50                                 Point 7                                                                             550      (60)    600    (35)  520    50                                 Point 8                                                                             600      55      600    35    540    (55)                               ______________________________________                                         Impact values without () = satisfactory, with no destruction of the film.     Impact values with () = not satisfactory, with destruction of the film        occurring                                                                     .sup.1) The value given indicates the product of the weight of the fallin     element (kg) and of the height of fall (cm).                             

Storage in water at 80° C. in mains water on 5 mm panel blasted withWheelabrator GE 40, SA 2.5, immersed in 10% strength by volume BasomatPT, preheated at 240° C., coated, after-baked at 240° C. for 120 sec.and immediately cooled in water. Coat thickness: 400-500 μm

    ______________________________________                                                       Comparison Comparison                                          Example 1      Example 1  Example 2                                           Hot         Cold   Hot     Cold Hot  Cold                                     ______________________________________                                        No exposure     +            *         *                                       168 hours                                                                            +       +      -     *    -    --                                      336 hours                                                                            +       +      -     *    -    -                                       504 hours                                                                            +       +      -     *    -    -    3t12vpsbl                          672 hours                                                                            +       +      -     -    -    -                                       840 hours                                                                            +/*     +      */-   */-  -    --                                     1008 hours                                                                            +/*     +/*    -     -    -    --                                     ______________________________________                                         ++ = very good, + = good, * = moderate, - = poor, -- = very poor         

CD test on 10 mm panels blasted with Wheelabrator GH 40, SA 2.5,immersed in 10% strength by volume Basomat PT, preheated at 240° C.,coated, after-baked at 240° C. for 120 sec. and immediately cooled inwater. Coat thickness: 400-500 μm

    ______________________________________                                        Submigration               Comparison                                                                            Comparison                                 after       Example 1      Example 1                                                                             Example 2                                  ______________________________________                                        30 days RT  0-1 mm         x       x                                           2 days 65° C.                                                                     0 mm           x       x                                          14 days 65° C.                                                                     1 mm           x       x                                          ______________________________________                                         x = poor adhesion, therefore not possible to determine submigration.     

Storage in water of the free film at 80° C. in mains water: Coatthickness: 400-500 μm

    ______________________________________                                                                Comparison                                                                              Comparison                                  Water uptake after                                                                         Example 1  Example 1 Example 2                                   ______________________________________                                         240 hours   5.20%       8.14%    6.01%                                        504 hours   5.86%       9.89%    8.82%                                        744 hours   6.21%      11.06%    9.28%                                       1008 hours   7.05%      12.22%    10.58%                                      ______________________________________                                    

We claim:
 1. Powder coating comprisingepoxy resin selected from thegroup consisting of epoxidized bisphenol A, epoxidized bisphenol F,epoxidized novolak resins and mixtures thereof, phenolic crosslinkingagents having methylol functionality, wherein the epoxy resins andphenolic crosslinking agents are employed in a ratio such the ratio ofepoxide groups to phenolic groups is about 1:1, catalysts, and fillerscomprising crystalline silicates functionalized with glycidyl groupspresent in an amount from 10 to 40% by weight, based on the total weightof the powder coating and wherein said coatings are resistant tocorrosion, hot water and cathodic delamination.
 2. Powder coatingaccording to claim 1, wherein the epoxy resin is an epoxidized novolakresins having an average functionality in the range from 2.4 to 2.8 andwith an epoxide equivalent weight in the range from 600 to
 850. 3.Powder coating according to claim 1, characterized in that the phenoliccrosslinking agents used are selected from the group consisting ofhydroxyl group-containing bisphenol A phenolic resins and hydroxyl groupcontaining bisphenol F phenolic resins having a hydroxyl equivalentweight in the range from 180 to
 600. 4. A process for improvingresistance of metal pipes to corrosion, hot water and cathodicdelamination, comprising, applying to metal pipe, a single-layerexterior coating of a powder coating according to claim
 1. 5. Metal pipeexterior coating comprising a powder coating according to claim
 1. 6. Apowder coating comprisingepoxy resin selected from the group consistingof epoxidized bisphenol A, epoxidized bisphenol F, epoxidized novolakresins and mixtures thereof, phenolic crosslinking agent having methylolfunctionality, wherein the epoxy resins and phenolic crosslinking agentsare employed in a ratio such the the ratio of epoxide groups to phenolicgroups is about 1:1, catalysts in an amount of from 0.001 to 10% byweight, based on the total weight of the epoxy resin and the phenoliccrosslinking agent, and fillers comprising crystalline silicatesfunctionalized with glycidyl groups present in an amount from 10 to 40%by weight, based on the total weight of the powder coating, said powdercoatings being resistant to corrosion, hot water and cathodicdelamination.